The present invention qenerally relates to a fluid-friction clutch and more particularly to a fluid-friction clutch in which the clutch torque can be varied.
In a fluid-friction clutch disclosed by German Patent Specification 3,529,234, the diametral dimensions of the clutch chamber, compensating chamber and adjusting piston are identical, so that the clutch chamber and compensating chamber are located on the same mid-radius relative to the clutch axis. Consequently, the friction discs are wetted by fluid because a fluid ring forms on the inner cylindrical surface of the entire clutch chamber under the effect of the centrifugal force. This wetting of the friction discs is accomplished even in the lifting Position of the adjusting piston, although the enclosed shearing gaps are then widened. Although the compensating chamber of the above-noted fluid-friction clutch is located on both end sides of the adjusting piston having axial Passages for this purpose, this design is nevertheless only adopted in order to make the clutch torque independent of pressure changes in the fluid as a result of the positive-displacement effect of the adjusting piston.
German Pat. Specification No. 3,539,484 discloses a fluid-friction clutch in which the clutch housing is not closed on all sides, but has an orifice. The orifice is provided in order to form two differential-pressure surfaces of equal size which act oppositely to one another on the adjusting piston which acts on the disc stack via a thrust ring. One differential-pressure surface is subjected to atmospheric pressure via the orifice with the assistance of a thrust spring and acts with the effect of widening the shearing gaps. The other differential-pressure surface located on the end face of the adjusting piston opposite the disc stack is subjected to the pressure of the viscous fluid via axial piston ducts. At an increased working temperature of the viscous fluid, this other differential-pressure surface actuates the disc stack to a block, thereby generating a mechanical friction state, so that the clutch slip becomes zero and the working temperature falls again to admissible operating values.
To form the differential-pressure surfaces, the disc carrier of the clutch housing rests against the inner surface of a sleeve-shaped housing insert which is arranged concentrically within the clutch housing and which is retained on the adjacent housing wall of the clutch housing fixedly in terms of thrust and movement by one end face. The adjusting piston, guided by a first sliding-gasket arrangement on the inner surface of the housing insert, passes through the open other end face of the housing insert, and at its piston end located outside the latter, has a piston part of widened diameter which is guided on the cylindrical inner surface of the clutch housing by a second sliding-gasket arrangement. The two inner surfaces interacting with the sliding-gasket arrangement limit the two differential-pressure surfaces.
In another fluid-friction clutch disclosed by German Pat. Specification No. 3,701,884, neither an emptying of the clutch chamber in the disconnected state nor resilient spacer parts are provided, so that in the disconnected state there always arises a fluid-friction moment which depends on the atmospheric pressure because of a thereby open connection between the clutch chamber and the atmosphere. Located in this open connection to the atmosphere is a working-pressure chamber which is constantly open to the clutch chamber and is provided in the shaft and which belongs to a displacement piston which, during its compression stroke, causes an increase of pressure in the clutch chamber by closing the respective orifice to the atmosphere, so that it is possible to increase the clutch torque up to the mechanical frictional connection of the clutch discs.
An object of the present invention is to provide a fluid-friction clutch with a disc stack and with an adjusting piston for varying shearing gaps between discs of the stack, in which the clutch torque can be varied by the position of the adjusting piston and becomes completely zero in a non-actuated lifting position of the adjusting piston.
In the fluid-friction clutch according to certain preferred embodiments of the present invention, during a lifting stroke of the adjusting piston, the viscous fluid escapes from the disc stack into a compensating chamber expanded by the movement of the adjusting piston and collects radially outside the disc stack due to the effect of the centrifugal force. As a result, the shearing surfaces of the discs are no longer reliably wetted by the viscous fluid and the clutch torque therefore becomes zero.
Such a fluid-friction clutch is consequently ABS-compatible, i.e., suitable for use as a transverse or longitudinal differential in a vehicle with four-wheel drive and automatic anti-lock control.
In certain preferred embodiments of the clutch according to the invention, the disc spacing and the filling ratio of the clutch chamber are varied. This can be obtained by a hydraulic connection, via which pressure oil is conveyed to the adjusting piston of the fluid-friction clutch. The adjusting piston, by being subjected to pressure oil, ensures that the discs are pushed together. At the same time, the filling ratio of the clutch is changed because oil is displaced out of the compensating chamber and into a working chamber of the disc stack. This compensating chamber can include pockets which are located in the clutch housing.
If a fluid-friction clutch of highly variable characteristics is designed so that, without any additional oil pressure, there is no wetting of the discs with silicone oil at all, a freewheeling can be obtained with the fluid-friction clutch according to certain preferred embodiments of the invention. With an increasing feed of pressure oil, silicone oil is forced between the discs and (depending on the design of the fluid-friction clutch) the discs are moved towards one another, that is to say there is both an increase in filling and a decrease in the spacing. These operative mechanisms occur simultaneously and cause an increase in the transmissible torque in this fluid-friction clutch.
The fluid-friction clutch according to certain preferred embodiments of the invention can be used as a cut-in clutch for a driving axle of a motor vehicle with four-wheel drive. Furthermore, the fluid-friction clutch according to preferred embodiments of the invention can replace an axle differential if it is regulated by electronics in such a way that the transmitted torque corresponds to the driving state.
The disconnection of the fluid-friction clutch according to preferred embodiments of the invention when the oil pressure is cancelled takes place via the effect of the centrifugal force, and furthermore small cup springs are arranged between the discs. In this arrangement these cup springs are preferably located between two identical discs.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.